Grotthuss shuttling of an excess proton charge defect through hydrogen
bonded water networks has long been the focus of theoretical and
experimental studies. In this work we show that there is a related
process in which water molecules move (shuttle) through a hydrated
excess proton charge defect in order to wet the path ahead for
subsequent proton charge migration. This process is illustrated through
reactive molecular dynamics simulations of proton transport through a
hydrophobic nanotube, which penetrates through a hydrophobic region.
Surprisingly, before the proton enters the nanotube, it starts shooting
water molecules into the otherwise dry space via Grotthuss shuttling,
effectively creating its own water wire where none existed before. As
the proton enters the nanotube (by 23 angstrom), it completes the
solvation process, transitioning the nanotube to the fully wet state. By
contrast, other monatomic cations (e.g., K+) have just the opposite
effect, by blocking the wetting process and making the nanotube even
drier. As the dry nanotube gradually becomes wet when the proton charge
defect enters it, the free energy barrier of proton permeation through
the tube via Grotthuss shuttling drops significantly. This finding
suggests that an important wetting mechanism may influence proton
translocation in biological systems, i.e., one in which protons create
their own water structures (water wires) in hydrophobic spaces (e.g.,
protein pores) before migrating through them. An existing water wire,
e.g., one seen in an X-ray crystal structure or MD simulations without
an explicit excess proton, is therefore not a requirement for protons to
transport through hydrophobic spaces.